In a wireless communications system, frames are sent over a wireless link so that a transmitter and a receiver can communicate. In such a system, each transmitter and each receiver implements a link layer window that keeps track of the frames sent and received between the two devices. The link layer window has knowledge of which frames are sent and which frames have been acknowledged as having been received. For example, if a transmitter has sent frames 1 2 3 then the link layer window at the transmitter has knowledge that it has sent frames 1 2 3. If the receiver has received frames 1 2 3, then the link layer window at the receiver has knowledge that frames 1 2 3 have been sent by the transmitter.
If all the frames that are sent are acknowledged, then the link between the receiver and transmitter is considered to be in synchronization. However, if the knowledge of frames sent (also termed the transmitter's link layer window) is different than the knowledge of frames received (also termed the receiver's link layer window), then the link layer windows of the receiver and transmitter are considered to be out of synchronization. For example, if a transmitter sends frames 1 2 3 (e.g. the transmitter's link layer window), but the receiver receives frame 2 (the receiver's link layer window) without receiving frame 1, the receiver has knowledge that the link layer windows are out of synchronization.
If the link layer windows become out of synchronization, the link layer windows at the receiver and transmitter need to be reset. Resetting the link layer windows requires the receiver to clear its link layer window, e.g. clear the link layer window of frame 2, and requires the receiver to send an over-the-air message that informs the transmitter that the receiver is not in synchronization with the transmitter. Upon the transmitter receiving the message, the transmitter clears the transmitter's link layer window and retransmits the frames that were in the transmitter's link layer window, e.g. frames 1 2 3 (also termed “in flight” frames).
Thus, resetting link layer windows between a receiver and transmitter requires actions such as having to perform additional over-the-air signaling, having to flush link layer windows, and having to retransmit frames. Such actions are inefficient to providing efficient communications between the transmitter and receiver. Accordingly, there is a need for improved synchronization of link layer windows.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.